Aluminum alloy electrode



United States Patent 3,240,688 ALUMINUM ALLOY ELECTRODE Michael J. Pryorand Douglas S. Keir, Hamden, and Philip R. Sperry, North Haven, Conn.,assignors to Olin Mathieson Chemical Corporation No Drawing. Filed Apr.21, 1964, Ser. No. 361,531 20 Claims. (Cl. 204-148) This application isa continuation-in-part of US. patent application Serial No. 251,024,filed January 14, 1963, now US. Patent 3,189,486, Serial No. 60,166,filed October 3, 1960, now US. Patent 3,180,728, Serial No. 171,114,filed February 5, 1962, now US. Patent 3,186,- 836, and Serial No.304,923, filed October 27, 1963.

The present invention relates to an improved aluminum alloy which may beadvantageously utilized in a number of applications. For example, theimproved alloy of the present invention may be advantageously utilizedin (l) a primary electric battery suitable for use with liquidelectrolytes, such as aqueous electrolytes and especially sea Water, and(2) as sacrificial aluminum anodes in conjunction with a metalliccathode which thereby receives substantial protection against corrosion.

Magnesium and magnesium alloys in the form of sheet are generally usedas the anodes of electric cells or batteries adapted to utilize seaWater or similar aqueous electrolytes. The cost of conventional seaWater batteries utilizing magnesium and magnesium alloys has been foundto be prohibitively high except for military applications Thisprohibitively high cost is due in part to the high price of magnesiumand also due to the difficulty in rolling the hexagonal metal down tolight gage sheet of less than 0.020 inch thickness.

Further disadvantages of magnesium and magnesium alloys for thisapplication include the fact that they generally corrode readily insaline mediums even When uncoupled. In addition, relatively low powerefliciency on the order of about 60 percent is obtained. Further, theyare accompanied by a marked hydrogen-evolution problem and arecharacterized by a power output which falls with time and for whichspecial design allowances must be made.

Zinc is disadvantageous, inter alia, as it provides insufficient poweroutput to be a useful anode material in this type of power cell.

The more widely used sacrificial anodes for protection of ferrousstructures against corrosion are the zinc and magnesium anodes. Aluminumalloys have not been as Widely adopted for this purpose as the zinc andmagnesium anodes because they have previously produced only lowprotective currents equivalent to those generated by zinc anodes but ata much higher unit cost. Furthermore, aluminum alloys have frequentlyshown the characteristic of becoming highly polarized due to theaccumulation of insoluble corrosion products so that ultimately littleuseful protective current is delivered. However, the magnesium depressesthe potential of steel in sea water into the hydrogen evolution rangeand stripping of protective coatings from the steel can result, forexample, paint coatings. Furthermore, magnesium itself produces copiousquantities of hydrogen when it serves an as anode in sea water. This isof particular significance in connection with protection of sea waterballast tanks in ships for which purpose magnesium anodes have beenfound to be hazardous. Zinc is undesirable due to the low galvanic icecurrents delivered which necessitates the use of a plurality of anodesin order to provide acceptable current levels.

It is therefore an object of the present invention to provide animproved aluminum alloy having a wide variety of uses, for example,which is capable of being utilized as a sacrificial anode and in animproved electric cell or battery adapted to utilize sea water or otherelectrolytes.

It is a further object of the present invention to provide an improvedbattery as aforesaid which attains high average current density, highpower output and high cur rent potential.

It is a still further object of the present invention to provide animproved battery as aforesaid which is inexpensive and economical whilestill attaining excellent results.

It is a still further object of the present invention to provide animproved cathodic protection system and an improved method ofcathodically protecting a ferrous metal structure in contact with amedium corrosive thereto.

Further objects and advantages of the present invention will appearhereinafter.

In accordance with the present invention it has now been found that theforegoing objects and advantages may be readily accomplished byproviding an aluminum base alloy containing at least percent aluminum,between 0.04 and 0.5 percent tin and from 0.005 to 1.0 percent gallium,with the tin being retained in solid solution to the maximum degree.

The alloy of the present invention is particularly useful forfabrication into a metal anode. The anode may be utilized in a primaryelectric cell containing, in addition to the anode, a consumableunpolarized cathode and a liquid-electrolyte. Still further, the anodemay be utilized in a cathodic protection system comprising a cathodicmetal structure and the aluminous sacrificial anode electricallyconnected thereto, with the metal structure and the anode being incontact with a medium corrosive to said metal structure.

The above identified co-pending application Serial No. 251,024 teachesthat a metal anode comprising an aluminum base alloy containing at least90 percent aluminum and between 0.04 and 0.5 percent tin, with the tinretained in solid solution to the maximum degree, is highly useful andadvantageous and in fact attains surprising advantages over previouslyknown systems. It has now been found that in accordance with the presentinvention still further improvements may be obtained by providing inaddition to the aluminum and tin alloy a specified quantity of gallium.The gallium provides greatly improved current density characteristicsover the alloy without the gallium present while retaining the highlydesirable and advantageous and in fact surprising characteristics of thealuminum-tin alloy of co-pending application Serial No. 251,024.

In addition, the aluminum-tin-gallium alloy of the present inventionprovides: (1) a maintenance of the current plateau at a nearlyconsistent level during most of the useful life of the cell rather thanthe continual decline of current experienced with the magnesium alloys;(2) markedly reduced hydrogen-evolution and less temperature rise. Thesecharacteristics greatly simplify battery design and operation.

An additional advantage of the alloys of the present invention is thatthey can be readily fabricated by casting by either hot or cold rolling,and can be readily rolled to small gages desirable for power cell anodesin distinction to magnesium where its hexagonal lattice severelyrestricts its fabricating.

The improved aluminum alloy of the present invention contains tin in anamount from 0.04 to 0.5 percent, at least 90 percent aluminum and from0.005 to 1.0 percent gallium, with the tin being retained in solidsolution to the maximum degree, i.e., about 0.1 percent with the excesstin or a suitable third ingredient being provided as taught inco-pending application, Serial No. 60,166, to improve uniformity ofcorrosion and to improve anodic efiiciencies.

The preferred manner of preparing this alloy is to heat the aluminum tinsample at elevated temperatures, e.g., around 550 to 630 C. with 620 C.being preferred, for a sufficient period of time to dissolve the maximumamount of tin and to redistribute excess tin or other alloying additionsin a coarse, particulate form which produces maximum uniformity ofattack and power efficiency. Generally, the heating period within thepreferred temperature range may vary between 15 minutes and 24 hours.After the heating period, the sample is cooled rapidly, for example, byimmersion in a large volume of water at ambient temperatures or in thecase of thin sheet, by cooling in air. For simplicity, this treatmentmay be termed homogenization treatment. Homogenization within the abovetemperature yields maximum tin in solid solid solution. Outside of thisrange the amount of tin in solid solution falls off markedly, thusyielding poorer electrochemical characteristics.

It has been found, as discussed in detail in the above co-pendingapplications, that the alloys of the present invention develop surfacelayers on oxidation of any portion thereof, which surface layers have anexcess of n-type defects in a concentration effective to substantiallyincrease the conductivity thereof.

The preferred amounts of tin are from 0.08 to 0.35 percent. In someinstances high purity aluminum may be preferred, for example, in theprimary cells; however, the present invention is not limited to the useof a high purity aluminum and may be prepared from lower purity aluminumcontaining up to 0.10 percent silicon and up to 0.1 percent iron. Itshould be naturally understood that the alloy of the present inventionmay contain in addition to the aluminum, tin, and gallium and incidentalimpurities, other metallic components which may be added to achieveparticularly desirable results.

Generally, insoluble elements may be added to the alloy, i.e., elementswhich have less than 0.03 percent maximum solid solubility in aluminum.The total amount of these insoluble elements should be no greater than0.5 percent. These insoluble elements have no significant effect oncurrent output as they do not reduce the solid solubility of tin inaluminum, but they act as second phase particulate cathodes and largeamounts ultimately reduce anodic efliciency by promoting local corrosionof the anode.

Soluble elements may also be added to the alloy. The soluble elementsmay be considered either lattice expanders or lattice contractors, i.e.,ternary addition elements which either expand or contract the aluminumlattice. Generally lattice expanders stabilize tin in retained solidsolution and permit high galvanic currents to be drawn from the alloy.Lattice expanders may be used in an amount from about 0.001 to 8percent, with typical lattice expanders and amounts thereof which may beused including: magnesium from about 0.001 to 7.0 percent which isparticularly preferred; zirconium from about 0.001 to 0.3 percent;bismuth from about 0.001 to 0.5 percent; indium from about 0.001 to 0.5percent; and mixtures thereof. It should be noted that gallium should beconsidered as a lattice expander since it stabilizes tin in retainedsolid solution and permits high galvanic currents to be drawn from thealloy.

Lattice contractors generally reject tin from solid solution, but smallamounts may be tolerated, for example, zinc up to 0.01 percent; copperup to 0.002 percent; silicon up to 0.10 percent; and manganese up to0.05 percent.

The primary cell of the present invention employs a consumableunpolarized cathode, a liquid electrolyte and the improved metal anodeof the present invention. As a cathode material any consumable andunpolarized cathode may be conveniently employed, and preferably areadily reducible and insoluble metal salt or oxide, for example, asilver salt or oxide or a copper salt or oxide, a catalyzed porouselectrode, such as porous metal or carbon wherein oxygen from without iscontinually consumed.

In the primary cell of the present invention it is preferred to utilizesolid, fused silver chloride as a cathode. Alternatively, any silversalt may be utilized as the cathode material, provided the salt is atleast as soluble as silver chloride, but sufficiently insoluble to avoiddisintegration of the cathode during operation of the cell. Among suchother cathodic materials which may be employed are silver oxide, silverchromate, silver sulfate, silver phosphate, silver acetate and silvercarbamate. Cells may be formed with cathodes of silver salts moreinsoluble than silver chloride such as silver bromide and silver iodide,but the voltage is considerably lower since the cathodic material ismuch more insoluble than the silver chloride. Exemplificative coppercompounds include preferably copper oxides.

The electrolytes which may be employed are broadly any liquidelectrolyte and preferably the liquid-aqueous type electrolytes. Theelectrolyte which should be employed should, in addition to being liquidat operating temperatures, be one which does not polarize the anode orthe cathode and one free from inhibitive action on the anode.

The primary cell of the present invention is especially adapted toutilizing sea water as the electrolyte; however, it is apparent that thecells and batteries of the present invention will operate advantageouslyin electrolytes other than sea water, for example, any aqueous solutionof sodium chloride may be conveniently employed, such as a 3.5 percentaqueous solution of sodium chloride. Similarly, other alkali metalchlorides or alkaline earth metal chloride will be satisfactory. Othersuitable electrolytes, weak or strong, dilute or concentrated may beconveniently employed. Water also yields an operative cell although aconsiderable time may be required before the cell reaches its fullcapacity. Exemplificative of the nonaqueous type electrolytes includefused sodium chloride or potassium chloride, including low meltingalkali halide eutectics.

Naturally, the primary cell of the present invention may be prepared byany of the conventional means well known in the art. In the preparationof the primary cell of the present invention, for example, the anode andcathode material may be separated or spaced apart by any conventionalmeans, for example, thin films of a chemically stable material such asnylon may be adherred to the anode material. If the particular cell orbattery under consideration is intended to operate at a high currentdensity, the electrodes should be more closely spaced. In a cell orbattery not intended to operate at high current densities, close spacingis not required. In the low current density batteries, rubber strips ortabs at the edges of the electrode sheets may be employed.

The cathode material may be prepared by any of the conventional means,for example, cast sheets of substantial thickness may be employed orrolled silver chloride may be produced by suspending a body of silver,such as silver screen, in a dilute chloride solution for a timesufficient to form a silver chloride coating of the desired thickness.Other means for preparing the cathodic material are well known in theart. It is preferred to set up a plurality of the primary cells spacedfrom one another so that individ ual cells are established between theplates of succeeding electrodes when immersed in an electrolyte.

' below.

the sheet.

, comprising a cathodic metal structure and at least one aluminoussacrificial anode electrically connected thereto, both the metalstructure and the anode being in contact with a medium corrosive to saidmetal structure, said anode comprising the above aluminum alloy of thepresent invention.

The anodes of the present invention can be used in cathodic protectionsystems for underground structures, such as pipe lines, foundations, andthe like. They may be used in fresh water or in saline aqueous media.They are particularly well suited for use in sea water, and provide forthe first time, cathodic protection systems for protection of iron, suchas ships hulls, ballast tanks, and commercial fishing devices, such aslobster pots, which are free from the shortcomings of previously usedsystems.

In carrying out the present invention, the sacrificial anode of the typepreviously described, is attached to a metal structure to be protected,such as, for example, a ferrous metal structure, by means of a suitableelectrical conductor, and then immersed or imbedded in the surroundingcorrosive medium, in accordance with the customary practice. The alloyanode may be of any desired shape or size, such as, for example, acylindrical piece, or a trapezoidal shaped member.

The present invention and improvements resulting therefrom will be morereadily apparent from a consideration of the following illustrativeexamples.

EXAMPLE I This example describes the preparation of the alloy of thepresent invention. The aluminum used was super purity aluminumcontaining 0.001 to 0.002 percent silicon and 0.002 and 0.003 percentiron. In all cases the alloying addition was pure tin and pure galliumwhere gallium was used.

The ingots were cast and after casting the ingots were rolled to sheetand were given a homogenization heat treatment of 1145 F. for one hourfollowed by rapidly air cooling in order to retain the tin in solidsolution to the maximum degree. All samples are degreased beforetesting.

' EXAMPLE II lnthe following example primary cells were prepared inidentical fashion except that the anodic material in each primary celldiffered in composition as indicated In all cases the anode material wasprepared as in Example I.

The test primary cells were constructed in the conventional manner. Theanodes were shaped from sheet material generally about 0.012 inch thick.The cathodes were shaped from silver chloride sheet about 0.015 inchthick. The cathodes were modified to incorporate insulating spacerswhich would allow them to be placed in close proximity to the anodeswithout electrical short circuiting. This was done, in one of theconventional art manners, by using glass beads fastened to one side of Amultiplicity of holes was drilled in the cathode to increase thereactive surface of the silver chloride. In addition, the silverchloride was partially reduced to silver in a conventional manner byimmersion in a photographic film developer.

The electrical contact with the silver chloride cathode was furnished bya 0.001 inch thick 99.9 percent pure silver foil held in pressurecontact in the conventional fashion. External electrical connectionswere made to the anode and cathode, suitably insulated from each otherand from the electrolyte. The anode, cathode and spacers were then tapedtogether to complete the cell structure.

The cell was activated by immersing it in a solution of 3.5 weightpercent sodium chloride in distilled water. The circuit was completedthrough a recording ammeter 6 which provided a 1 ohm external loadresistance for the cell.

The results are shown in the following table.

The foregoing experiments demonstrate that small amounts of gallium, forexample, as low as 0.01 percent, gives a significant and impressiveincrease in current density and that a peak in the increase is obtainedat about 0.05 percent gallium. The alloys with gallium continued todisplay the other favorable characteristics associated with aluminum-tinalloys as shown in the above co-pending applications, such as low gasevolution and evenly maintained level of current output over the usefulperiod of operation of the cell. In addition, the alloys with galliumexhibited good current rise characteristics.

EXAMPLE III In the following example the tests of Example II wererepeated with additional anode materials except that the time in secondsto reach peak current was also measured. In all cases the anode materialwas prepared as in Ex ample I.

The results are shown in the following table.

This example in addition to showing the same effect as Example II showsthat the lattice expander, magnesium, increases the maximum current overthe anode without magnesium and simultaneously shows a markedimprovement in the current rise characteristics. The presence of themagnesium does not detract from the effect of the gallium with respectto current density and improves the current rise characteristics.

EXAMPLE IV The following example illustrates the effect of providing thetin component in solid solution to the maximum degree in the alloy ofthe present invention.

All samples were prepared in a manner after Example I except that thehomogenization temperature varied as shown in the table below. In everycase the alloy had the following composition: gallium, 0.05 percent;tin, 0.20 percent; magnesium, 0.5 percent, and the balance essentiallyaluminum. The current density characteristics were measured.

The results are shown in the table below.

Table III Homogenization Peak Current Temperature Density (amp/sq. in.)

1. 37 620 C 1. 40 1. 41 1. 40 565 C 1. 40 l' a 540 C 28 This exampledemonstrates the criticality of providing tin in solid solution to themaximum degree. With the sample homogenized at 540 C. the tin was notprovided in solid solution to the maximum degree and much poorercharacteristics were shown. Further, a sample without any homogenizationat all shows still poorer characteristics.

EXAMPLE V Table IV Composition (percent) Coulombs Flow- Alloy ing in 48hours Sn Fe 87 7 1,374,1,400,1,554 Nil This invention may be embodied inother forms or carried out in other ways without departing from thespirit or essential characteristics thereof. The present embodiment istherefore to be considered as in all respects illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims, and all changes which come within the meaning and range ofequivalency are intended to be embraced therein.

What is claimed is:

1. An aluminum base alloy consisting essentially of at least 90 percentaluminum, between 0.04 and 0.5 percent tin and between 0.005 and 1.0percent gallium, with the tin being retained in solid solution to themaximum degree, said maximum degree being 0.1 percent.

2. An alloy according to claim 1 wherein the tin is present in an amountfrom 0.08 to 0.35 percent.

3. An alloy according to claim 1 containing silicon in an amount up to0.10 percent and iron in an amount up to 0.10 percent.

4. An alloy according to claim 1 containing from 0.001 to 8.0 percent ofa lattice expander which has greater than 0.03 percent maximum solidsolubility in aluminum.

5. An alloy according to claim 4 wherein said lattice expander isselected from the group consisting of magnesium in an amount from 0.001to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent,bismuth in an amount from 0.001 to 0.5 percent, indium in an '8 amountfrom 0.001 to 0.5 percent and mixtures thereof.

6. A metal anode comprising an aluminum base alloy consistingessentially of at least percent aluminum, between 0.04 and 0.5 percenttin and between 0.005 and 1.0 percent gallium, with the tin beingretained in solid solution at a maximum degree, said maximum degreebeing 0.1 percent.

7. An anode according to claim 6 wherein said alloy contains tin in anamount from 0.08 to 0.35 percent.

8. An anode according to claim 6 wherein said alloy contains silicon inan amount up to 0.10 percent and iron in an amount up to 0.10 percent.

9. An anode according to claim 6 wherein said alloy contains from 0.001to 8.0 percent of a lattice expander which has greater than 0.03 percentmaximum solid solubility in aluminum.

10. An anode according to claim 9 wherein said lattice expander isselected from the group consisting of magnesium in an amount from 0.001to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent,bismuth in an amount from 0.001 to 0.5 percent, indium in an amount from0.001 to 0.5 percent and mixtures thereof.

11. In a primary electric cell containing a metal anode, a consumableunpolarized cathode and a liquid electrolyte, the improvement of a metalanode comprising an aluminum base alloy consisting essentially of atleast 90 percent aluminum, between 0.04 and 0.5 percent tin and between0.005 and 0.1 percent gallium, with the tin being retained in solidsolution to the maximum degree, said maximum degree being 0.1 percent.

12. A primary electric cell comprising a consumable, unpolarized cathodeselected from the group consisting of a silver salt, a silver oxide, acopper salt and a copper oxide, an aqueous electrolyte and a metal anodecomprising an aluminum base alloy consisting essentially of at least 90percent aluminum, between 0.04 and 0.5 percent tin and between 0.005 and1.0 percent gallium, with the tin being retained in solid solution tothe maximum degree, said maximum degree being 0.1 percent.

13. A primary electric cell according to claim 11 wherein theelectrolyte is sea water.

14. A primary electric cell according to claim 11 wherein said aluminumbase alloy contains from 0.001 to 8.0 percent of a lattice expanderwhich has greater than 0.03 percent maximum solid solubility inaluminum.

15. A primary electric cell according to claim 14 wherein said latticeexpander is selected from the group consisting of magnesium in an amountfrom 0.001 to 7.0 percent, zirconium in an amount from 0.001 to 0.3percent, bismuth in an amount from 0.001 to 0.5 percent, indium in anamount from 0.001 to 0.5 percent and mixtures thereof.

16. A cathodic protection system comprising a cathodic metal structureand at least one aluminous sacrificial anode electrically connectedthereto, both the metal structure and the anode being in contact with ametal corrosive to said metal structure, said anode comprising analuminum base alloy consisting essentially of at least 90 percentaluminum, between 0.04 to 0.5 percent tin and between 0.005 and 1.0percent gallium with the tin being retained in solid solution to themaximum degree, said maximum degree being 0.1 percent.

17. A cathodic protection system according to claim 16 wherein saidanode contains tin in an amount from 0.08 to 0.35 percent.

18. A cathodic protection system according to claim 16 wherein saidanode contains from 0.001 to 8.0 percent of a lattice expander which hasgreater than 0.03 percent maximum solid solubility in aluminum.

19. A cathodic protection system according to claim 18 wherein saidlattice expander is selected from the group consisting of magnesium inan amount from 0.001 to 7.0 percent, zirconium in an amount from 0.001to 0.3 percent,'bismuth in an amount from 0,001 to 0.5

percent, indium in an amount from 0.001 to 0.5 percent and mixturesthereof.

20. The method of cathodically protecting a ferrous metal structure incontact with a medium corrosive thereto which comprises: connecting tosaid metal structure an aluminous sacrificial anode and immersing saidanode in said corrosive medium, said anode comprising an aluminum basealloy consisting essentially of at least 90 percent aluminum, between0.04 to 0.5 percent tin and between 0.005 and 0.1 percent gallium, withthe tin 10 References Cited by the Examiner UNITED STATES PATENTS2,565,544 8/1951 Brown 204-148 2,886,432 5/1959 Schmitt et a1 751383,063,832 11/1962 Snyder 75138 3,172,760 3/1965 Sakano et al. 75-138JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

1. AN ALUMINUM BASE ALLOY CONSISTING ESSENTIALLY OF AT LEAST 90 PERCENT ALUMINUM, BETWEEN 0.04 AND 0.5 PERCENT TIN AND BETWEEN 0.005 AND 1.0 PERCENT GALLIUM, WITH THE TIN BEING RETAINED IN SOLID SOLUTION TO THE MAXIMUM DEGREE, SAID MAXIMUM DEGREE BEING 0.1 PERCENT. 